The invention relates generally to methods and apparatus for forming polygons for use in trapping in digital document preparation or prepress operations.
A page in an electronic document may include various types of objects, including text, line art, and images. Electronic documents are generally created by computer programs (also called application programs or simply applications) that may be executed by a user on a computer to create and edit electronic documents and to produce (directly or indirectly) printed output defined by the documents. Such programs include the Adobe Illustrator(copyright) and Photoshop(copyright) products, both available from Adobe Systems Incorporated of San Jose, Calif. Objects in electronic documents may be represented in vector form, raster form, or in hybrid forms.
A color page in an electronic document includes information about the colors used in the page. Colors are defined in accordance with a xe2x80x9ccolor spacexe2x80x9d, which provides a data representation of a range of colors in terms of basic color components. The specific color components will vary depending on the color system used. For example, in the CMYK color system, colors are represented as combinations of cyan (C), magenta (M), yellow (Y), and black (or xe2x80x9ckeyxe2x80x9d) (K).
To create a physical page printed with inks, data representations of colors are used to create color separations. This is generally done by computer programs running on general or special purpose systems. Generally, each color separation used by an output device will correspond to a color component of the color system used by the device. For example, data representations of colors in output generated for an imagesetter using a CMYK color system will be used to create color separations for cyan, magenta, yellow, and black, with each separation indicating regions where the corresponding ink should be applied, as well as the amount of ink that should be applied.
Misregistration or inaccuracies in the physical placement of two or more colors with respect to each other on a printed sheet in printing can cause unintentional gaps or overlaps at edges of color regions on an output page. Misregistration may occur for a variety of reasons relating to the performance of people, machines, and materials.
To minimize the effects of misregistration, a technique known as trapping adjusts the shapes of color regions by spreading (expanding) some color regions to prevent gaps, and choking (contracting) other color regions to prevent overlaps. In determining whether an edge of a shape requires a trap, trapping entails evaluating one or more trap conditions, such as a maximum color difference. Trapping also considers a trap width, defining a general distance around an edge where trapping conditions apply. The adjusted areas into which inks will be spread or from which inks will be contracted are referred to as xe2x80x9ctrap regionsxe2x80x9d and defined by a trap polygon. Trapping also entails determination of the amount of ink to be applied to the trap regions for each affected separation. Thus, each trap region has a trap color defining a color for the trap polygon.
When multiple objects form edges that are in close proximity, a trap polygon generated by conventional trapping operations for one or more of these objects can overlap another trap polygon or an object. This overlap may create undesirable printing effects and the trap polygons may no longer be invisible, defeating the original purpose of the trapping operation.
The invention provides methods and apparatus implementing a technique for forming a trap polygon which does not interfere with the print quality of other trap polygons or objects. Edges in close proximity to a color transition edge (xe2x80x9cCTExe2x80x9d) are checked for potential interference. A trap polygon is formed for the CTE that avoids any interfering edges or trap polygons for those edges. The technique can be applied to form a trap polygon for each edge in a page to be printed.
In general, in one aspect, forming a trap polygon for trapping a color transition edge, where the trap polygon has an associated trap color determined by colors defining the color transition edge, includes: identifying one or more potentially interfering edges which intersect a keep away zone defined by the color transition edge; screening the potentially interfering edges to identify interfering edges which are part of an object which is of a significantly different color than the trap color; and forming a trap polygon for trapping the color transition edge including shaping the trap polygon to avoid overlapping objects and any trap polygons corresponding to any interfering edges.
The invention provides one or more of the following advantages. Overlapping trap polygons are avoided improving print quality. Similarly, trap polygons can be generated that do not overlap other graphical objects in a page. Portions of a trap polygon which do not aid in trapping are removed while shaping the trap polygon. Trapping can be performed with large trap widths improving print quality for applications such as printing on a medium other than paper. The invention also provides a smooth transition between abutting trap polygons.